To cut to the chase: fanworks are being copied, probably bot-scraped, from AO3 and potentially other sites and listed on ebooks-tree.com. You can search the site to see if you have been affected.
OTW/AO3 has an advice page on their Tumblr:
Also, feng_shui_house has some additional suggestions on their DW:
Additional links, helpful advice, please add to comments.
ETA: if someone on the relevant Facebook groups could post the AO3 advice link there, that might help people to focus their thinking. I can't do it where I am right now.
ETA2: Special Edition of Metanews (DW) http://metanews.dreamwidth.org/34536.ht
A new AO3 announcement: